Last sprinkler in zone has little pressure

The last sprinkler on the final zone of my 1 year old 6 zone sprinkler system is very very weak. It was working earlier in the day and then it wouldn't pop up fully so I unscrewed the rainbird, got the dirt out and put it back in, but now the pressure is weaker than before. Is it possible a rock or debris floated into the flex pipe while I was away? That just doesn't sound right. If there's a leak/crack further up the line I'm not sure how it would happen since there was no digging.

When I turn the zone on without the sprinkler head attached water flows out very slowly and I don't see any puddling before the flex pipe. I tried unscrewing the valves in the control box, but that didn't do anything. The other 3 heads before the last head work perfect. I also can't find any bubbling in the lawn incase there is a leak after the 3rd head. Is there an easy way to blow out any possible debris without the winterizing process?

I know nothing was very clear there, but hopefully someone has come across this. Thanks

You might have a constriction in the pipe caused by tree roots. Squeeze a (poly) pipe from the outside, and flow is choked off.
The strongest indication for this is pressure. Choke off one head, and the other heads on the same zone will now have higher pressure.

I think i'll need to dig. there aren't any trees near by and it's a brand new development so everything has been cleared. It's along the driveway and the last head is on the other side of the sidewalk so as long as the crack isn't directly underneath the sidewalk it should be relatively simple. I'll keep everyone updated with the cause in case this happens to someone else in the future